High temperature gradient elution liquid chromatography

ABSTRACT

In a liquid chromatography commercial process for the separation of two or more components contained in admixture in a feedstream including the steps of (a) contacting the feedstream mixture containing the components to be separated with a bed of crystalline aluminosilicate adsorbent at conditions to effect the selective retention of two or more of the components by said adsorbent, (b) passing through said bed an eluent selected from the group consisting of aromatics and substituted aromatics, (c) increasing the strength of the eluting agent in the liquid carrier during the operation of the steps (a) and (b) above, (d) recovering from said bed a stream or streams containing at least a portion of the less selectively adsorbed components, and (e) recovering a stream substantially enhanced in concentration of said selectively adsorbed components relative to other components of feedstream wherein the improvement comprises operating the process at temperatures of from 160*C to about 200*C to thereby effect the desired separation under conditions that cause low elution volume to feed ratios. The improved liquid chromatography process is particularly applicable to the separation of paraxylene and ethylbenzene from a C8 aromatic isomer mixture containing ethylbenzene. The above process when operated at temperatures of about 160*C or higher, and using larger feed slug sizes to provide lower elution volume to feed ratios thereby produces paraxylene more economically.

United States Patent Geissler Sept. 2, 1975 HIGH TEMPERATURE GRADIENT ELUTION LIQUID CHROMATOGRAPHY Primary Examiner-Delbert E. Gantz Assistant Examiner-C. E. Spresser Attorney, Agent, or Firm-F. A. Santoro [57] ABSTRACT In a liquid chromatography commercial process for the separation of two or more components contained in admixture in a feedstream including the steps of (a) contacting the feedstream mixture containing the components to'be separated with a bed of crystalline aluminosilicate adsorbent at conditions to effect the selective retention of two or more of the components by said adsorbent, (12) passing through said bed an eluent selected from the group consisting of aromatics and substituted aromatics, (c) increasing the strength of the eluting agent in the liquid carrier during the operation of the steps (a) and (b) above, (d) recovering from said bed a stream or streams containing at least a portion of the less selectively adsorbed components, and (e) recovering a stream substantially enhanced in concentration of said selectively adsorbed components relative to other components of feedstream wherein the improvement comprises operating the process at temperatures of from 160C to about 200C to thereby effect the desired separation under conditions that cause low elution volume to feed ratios. The improved liquidchromatography process is particularly applicable to the separation of paraxylene and ethylbenzene from a C aromatic isomer mixture containing ethylbenzene. The above process when operated at temperatures of about 160C or higher, and using larger feed slug sizes to provide lower elution volume to feed ratios thereby produces paraxylene more economically.

8 Claims, 2 Drawing Figures mxm XYLENE rm RECOVERY COLUMN STORAGE 6 DRUM n f\ 5 I\ 9 g? ETHYL BENZENE l RECOVERY COLUMN STORAGE DRUM CHROMATO- GRAPHIC COLUMN \K 16 f s 3 PARAXYLENE RECOVERY COLUMN 1 PATENTEU SEP 2 I975 SHEET 2 BF 2 Figure 2 ELUTION CHROMATOGRAPHY PROCESS WITH GRADIENT 2238 mw m v wzwfixfi m v 2 2238 9536 652810 23% ww w 2238 598mm 5 m HIGH TEMPERATURE GRADIENT ELUTION LIQUID CHROMATOGRAPHY FIELD OF THE INVENTION This invention relates to an improvement in the operation of commercial liquid chromatographic separation processes of mixtures into two or more components. More particularly the invention relates to an improved elution chromatographic separation process carried out in either two columns or in a single column. The separation of a C, aromatic isomer mixture is particularly adaptable to the operation of the present invention.

BACKGROUND OF THE INVENTION In the liquid chromatographic separation of mixtures into two or more components on a commercial scale, it is economically desirable to operate under conditions that maintain low elution volume to feed ratios. The reason for this is mainly due to the cost involved in distilling the carrier and cluent from the feed components after they have been eluted from the chromatographic column; this cost increasing as the elution volume to feed ratio increases. One of the main causes of high elution volumes in these processes is related to the phenomena that peaks which are eluted last from the column have larger half widths than those eluted early.

Chromatography is a process whereby components contained in a fluid mixture may be separated from the mixture. This is accomplished by the selective retardation of one or more of the components of the fluid mixture as the fluid uniformly moves through a column containing a stationary substrate. The retarding results from the distribution of the components of the mixture between the substrate and the bulk fluid as the fluid moves past the stationary phase.

One of the particular chromatographic methods employed is called elution chromatography. In this technique. the feed mixture and carrier fluid and cluent are passed sequentially through the column. In this specification. carrier fluid is taken as liquid materials which are not significantly absorbed by the substrate in the presence of feed mixture components. Eluent is a term to describe liquid materials which are adsorbed by the substrate and compete for adsorption sites with the feed components. This sequential passage of carriereluent and feed leads to a differential migration of the feeds components according to their distribution between two phases. If the components of the sample have different distribution coefficients. a separation of the components is achieved as the components will elute in sequence from the end of the stationary phase. In ordinary elution development. there is a small range of retention volumes or retardation factors for optimum separation. The distribution coefficients must be sufficiently large so that the components eluted early are not pushed off the column as an unresolved series of bands. yet the distribution coefficients must be reasonably small if excessive elution times and some peak broadening are to be avoided. In addition. there must be a difference in the distribution coefficients of the components in order to effect their separation.

In prior copending application Ser. Nov 317.] 18. now IRS. Pat. No. 1835.043. a technique for the automatic gradual attainment of the elution power required for each components separation was described. It has now been discovered that by operating this technique at higher temperatures, smaller elution volumes are obtained and larger feed slug sizes can be employed to lower total elution volume to feed ratios. thus providing a process which produces paraxylene more economically.

DESCRIPTION OF THE PRIOR ART It is known in the separation art that certain adsorbents generally comprising cyrstalline aluminosilicates can be utilized to separate certain hydrocarbons from feed mixtures. In aromatic hydrocarbons separation, in particular the separation of the C, aromatic hydrocarbons, it is generally recognized that particular crystalline aluminosilicates are useful for a given C aromatic hydrocarbon. Illustrative of the prior art is Neuzil U.S. Pat. No. 3,55 8,732. which teaches an improved adsorptive separationprocess, the improvement comprising employing toluene as a desorbent.

Other art workers have taught the use of different kinds of X andY type zeolites for the separation of paraxylene from a mixture of C aromatic isomers. see for example. Neuzil U.s. Pat. No. 3,626,020. In U.S. Pat. No. 3,636.l2l to Stine et al. a dual adsorption and isomerization process for the recovery and isomerization of various C, aromatic isomers is described and diethylbenzene is the eluent preferred for operation of the process.

SUMMARY OF THE INVENTION The present invention is directed to an improved onecolumn liquid chromatographic separation process for C aromatic mixtures. The process is particularly useful and is. therefore, directed to the ultimate recovcry of pure paraxylene or paraxylene and ethylbenzene in purities exceeding 99% and in which 90% of the paraxylene introduced as feed is recovered.

The process utilizes crystal aluminosilicate zeolite sieves which are known to selectively adsorb paraxylene and ethylbenzene in preference to the other isomers. Sieves such as potassium exchanged type Y (KY). potassium ammonium exchanged type Y (ammonium KY) and barium potassium exchanged type Y. i.e.. (BaK)Y. These sieves are prepared by techniques described in U.S. Pat. No. 3.696.107. said disclosure herein incorporated by reference. The preferred sieve is KY sieve when either paraxylene alone or paraxylene and ethylbenzene are sought as products. The process includes the utilization of process steps such as contacting the feed mixture with. for example. a KY sieve under conditions to effect the preferential adsorption of paraxylene and ethylbenzene; contacting the bed of KY sieve containing said selectively adsorbed paraxylene and ethylbenzene with a carriercluent stream thereby affecting desorption of the adsorbed components from the bed material; recovering thereafter from the bed of KY sieve a stream or streams containing metaxylene. orthoxylene. paraxylene, ethyl benzene and mixtures thereof and ultimately recovering paraxylene and ethylbenzene as product streams.

In operating the abovedescribed process. as has been previously described in application Scr. No. 3 l 7.] 18. a means of reducing and thereby minimizing total elution volume to feed ratios has been found. In actuality. the operation of the aforesaid technique results in condensing the size of the overall chromatogram by decreasing tailing effects of the more strongly adsorbed components. The technique thus described is carried out by means of gradient elution chromatography; which may be operated in one of three ways. In the first method, the eluent concentration in the carriereluent is increased in a stepwise manner with time; in a second way, gradient elution is carried out by a con tinuous increase in the eluent concentration in the carrier eluent with time as the feed slug proceeds through the bed. Finally, in a third, but less preferred method, gradient elution is conducted by substituting for a weak eluent with a strong eluent during the elution of the feed components; such a substitution can take place in either the stepwise or continuous manner as described above.

It has now been discovered that when the abovedescribed process is carried out at temperatures of at least 160C. and larger feed slug sizes are employed, there is an overall 40% reduction obtainable in the elution volume to feed ratio which results from both an increase in feed slug size and decrease in the elution volume. Increasing the temperature of the bed during the operation of the gradient elution process condenses the chromatogram and pushes the peaks closer together. Operation of the process at the increased temperature while decreasing elution volume only slightly decreases ethylbenzene recovery because of the overlap of the ethylbenzene peak with the metaxylene/orthoxylene peaks. However, what is surprising is that the decrease in elution volume to feed ratio far outweighs any debit that must be taken for the decrease found in ethylbenzene recovery.

The present invention is operable with a number of different carrier-eluent compositions, preferably, however, metadiisopropylbenzene or paradiisopropylbenzene or their mixtures are used as eluting agents in the carrier eluent stream comprising from to 100% of the eluting agent, the remainder being carrier fluid. The carrier may be any inert diluent such as paraffins having carbon atoms from C to C Non-limiting representative examples of paraffins useful as diluents are hexane, heptane, isooctane, cetane and eicosane and the like. Typically employed is n-dodecane and a typical carrier-eluent stream is a composition of about metadiisopropylbenzene and 75% n-dodecane. In operating the gradient elution technique, the beginning 25% concentration of the eluting agent in the carrier-eluent may be increased to a final concentration of up to about 95%.

The present invention is operable under a wide range of conditions. For example, pressures are not critical and the present process may be operated at pressures in the range of from 0.1 to atmospheres, preferably from 0.1 to 10 atmospheres, most preferably, from 1 to 5 atmospheres. In applying the present invention to commercial liquid chromatographic separations of the C aromatic isomers particular conditions employed, such as optimum eluent, length of column, flow rate, sieve, etc., will vary from case to case.

However, in the operation of the present invention it has been found that a feed slug size of about 2.5 ccs is optimum when a 9 foot long, 0.18 inches i.d. column packed with 20-40 mesh K-Y sieve, m-diisopropylbenzene/dodecane as carriereluent is employed at a flow rate of0.3 feet per minute and at a temperature of from 160C. to about 200C. The resultant process demonstrates that the efficiency with which paraxylene is recovered is greatly improved.

The present process is also operable with or without recycle where recycle is defined as the amount ofa material recycled divided by the fresh feed times 100. Recycling can be in the range of from 10 to 100%, preferably from about 10 to Moreover, the process while in a preferred manner, is described as an improvement for the single column liquid chromatographic process, the operation of the various embodiments described herein are also useful as an improvement over the two column scheme defined in copending application Ser. No. 282,983, filed Aug. 23, 1972, now US. Pat. No. 3,843,518. wherein critical feed slugs sizes are utilized of from at least twice, preferably at least four times, the maximum feed slug employed to obtain three pure components in a single column scheme.

In a typical processing scheme, the gradient elution technique for increasing the strength of the eluting agent in a liquid carrier is carried out, for example, by increasing in a stepwise manner from 25 to the amount of eluting agent in the carrier. The process is carried out by utilizing a column containing a KY sieve through which is passed a carrier eluent composed of, for example, 20% metadiisopropylbenzene and 80% n-dodecane at a linear velocity of 0.3 feet per minute at a column temperature of 160C. The carrier eluent flow is stopped after the column has been sufficiently purged and a feed slug composed of a 20% paraxylene, 20% orthoxylene, 20% ethylbenzene and 40% metaxylene is injected into the column. The chromatograph development showing the separation of components is illustrated in FIG. 1. After feed slug injection, the carrier-eluent (D flow is immediately resumed; the composition of the carrier-eluent stream is changed to D in a stepwise manner by means of a threeport, two-way valve from a separate storage vessel containing m-DIPB. The change in composition of the carriereluent stream to D containing higher concentrations of eluent than D is effected at a definite time period after the column composition profile of FIG. 1A is obtained. The effect of D on the elution development of the feed slug in the column is illustrated in FIG. 1B and it is shown that p-xylene is eluted (desorbed) more quickly, while m-xylene and o-xylene are removed as product stream. A definitive volume of D is passed through the column and is followed by subsequent feed slug injection including utilization of carrier-eluent stream D The profile of the column composition as shown in FIG. 1C illustrates the removal of ethylbenzene as a product stream; FIG. lD shows p-xylene removed as product stream together with desorbent D The definite time period referred to above, where D is changed to D is a function of temperature, flow rate, column size, desorbent, and sieve. Said time period must be chosen such that the p-xylene peak elutes from the column predominantly in the presence of desorbent D 7 When the gradient elution technique is being utilized by means ofa continuous increase in the concentration of the eluting agent in the carrier-eluent stream, a typi-' cal processing scheme will be carried out similar to the scheme described above except that instead of stepwise transition from D to D the initial composition of carrier-eluent D is changed continuously until composi tion D is obtained. This may be carried out by conventional means such as the use ofa mixing tee and varying flow rates of two streams D,and D to produce any desired change from D to D The third gradient elution method or substitution technique is conducted similar to the schemes described above except that D is 2.5-100% ofa strong eluent such as ODCB while D is -10071 ofa weak eluent such as mDIPB or p-DIPB.

The present invention will be more easily understood by reference to FIG. 2.

In FIG. 2, representing a typical commercialscale application of the invention, the feed slug. stream 1, is introduced into a chromatographic column. The feed slug is followed by weak desorbent, stream 14. Next, the weak desorbent is followed by strong desorbent, stream 15. The strong desorbent is introduced at such a time and for such a duration that the most strongly held component of the feed (i.e. paraxylene) is eluted in a stream containing essentially only strong desorbent. Product recovery is achieved by directing the eluted streams to separate fractionation facilities. Stream 2 (first cut) is fractionated into a mixed xylene product (Stream 7), a weak desorbent bottoms (Stream 5) and a strong desorbent sidestream (Stream 6). The desorbent streams are sent to storage drums for reuse. Stream 3 (second cut) is fractionated into ethylbenzene product (Stream (9) and a weak desorbent bottoms (Stream 8). Stream 4 is fractionated into paraxylene product (Stream l3) and a strong desorbent bottoms (Stream 12). For the fractionation scheme shown, both the weak and strong desorbents must have higher boiling points than the products, and the strong desorbent must have a lower boiling point than the weak desorbent. This is true for 100% metadiisopropylbenzene as the strong desorbent and a mixture of metadiisopropylbenzene and tridecane as the weak desorbent.

To further illustrate the improved process of the present invention, the following examples are provided, however, it is to be understood that the details thereof are not to be regarded as limitations, as they may be varied as will be understood by one skilled in the art.

EXAMPLE 1 Potassium Y sieve was ground to 20-40 mesh and about grams were loaded into a 9 foot long, 0.25 inch O.D.. 0.18 inch l.D. stainless steel column. A carrier-cluent mixture of 25% m-diisopropylbenzene and 75% nC paraffin was fed through the column at 130C. at a flow rate of 1.5. The carrier-eluent flow was stopped and a 2.0 cc sample of 2071 paraxylene. 20% ethylbenzene, 2071 orthoxylene and 40% metaxylene was injected into the system upstream of the packed column through a sixport sample loop valve. Carrierelucnt flow was restarted immediately and samples of the stream eluting from the end of the column were taken periodically. Twenty minutes after the feed was injected a carrier-eluent of 10071 m-diisopropylbenzene was fed through the column at 130C at a flow rate of 1.5.

Each sample taken was analyzed for the weight per cent of the C, aromatics by gas chromatography. Results showed that pure cthylbenzene (free of other C isomers) was recovered and that the total elution volume for the C aromatic isomers was 90 cc.

EXAMPLE 2 The procedure of Example 1 is repeated except that a 2.5 cc, sample of 20/20/20/40 paraxylene- -ethylbenzene-orthoxylene-metaxylene is injected. The

elution volume to feed ratio is decreased. to 36 and the ethylbenzene recovery is decreased from 30% to 15% of the total ethylbenzene injected.

EXAMPLE 3 The procedure of Example 2 is repeated except that 160C is used as the column temperature. Results show that the elution volume is decreased to 64 cc and the elution volume to feed ratio is decreased to 25.6. The ethylbenzene recovery is only slightly decreased to 10% of the total ethylbenzene injected.

EXAMPLE 4 The procedure of Example 3 is repeated except that a metadiisopropylbenzene 25% paradiisopropylbenzene is substituted for the carrier-eluent stream. Substantially the same results as obtained in Example 3 are reproduced.

In each of Examples l-4 above inclusive, paraxylene recovery exceeded of paraxylene introduced as feed. 1

What is claimed is:

1. In a liquid chromatographic separation process of up to two components contained in admixture in a feedstream including the steps of:

a. contacting the feedstream mixture, wherein said feedstream mixture is a C aromatic feedstream containing ethylbenzene, with a bed of crystalline aluminosilicate adsorbent at conditions to effect the selective retention of up to two of the components by said adsorbent;

b. passing through said bed an eluent selected from the group consisting of aromatics and substituted aromatics;

c. increasing the strength of the eluent during the operation of step (b) above to thereby effect desired separation under conditions that cause low elution volume to feed ratios;

d. recovering from said bed a stream or streams containing a portion of the less preferentially adsorbed components and e. recovering a stream or streams substantially enhanced in concentration of paraxylene and ethylbenzene relative to other feedstream components wherein the improvement comprises operating the process at temperatures of from about 160C to about 200C to thereby effect the desired separation of one of said preferentially adsorbed components under conditions resulting in low elution volume to feed ratios without a corresponding substantial decrease in the separation of the other of said preferentially adsorbed components.

2. The process of claim 1 wherein the initial concentration of said eluent is about 25% and is increased to a final concentration in the range of from about 50 to about during the operation of steps (b) and (c).

3. In a liquid chromatographic process for the separation of paraxylene from a feedstream containing a C, aromatic isomer mixture and ethylbenzene including the steps of:

a. contacting said feedstream with a bed of crystalline aluminosilicate adsorbent at conditions to effect the selective retention of paraxylene by said adsor bent;

b. passing through said bed an eluent selected from the group consisting of aromatics and substituted aromatics;

c. increasing the strength of the eluent in a liquid carrier containing said eluent during operation of step (b) above;

d. recovering from said bed a stream or streams containing a portion of the less preferentially adsorbed feedstream components; and

e. recovering a stream or streams substantially enhanced in concentration of paraxylene wherein the improvement comprises operating the process at temperatures of from l60C to about 200C to thereby effect the desired separation of paraxylene under conditions resulting in low elution volume to feed ratios without a corresponding substantial decrease in the separation of ethylbenzene.

4. In a liquid chromatographic separation process for paradiisopropylbenzene and toluene;

c. increasing the strength of the eluent in the liquid carrier containing said eluent during the operation of step (b) above;

d. recovering from said bed a stream or streams containing a portion of the feedstream components less preferentially adsorbed by said sieve;

e. recovering a stream or streams substantially enhanced in concentration of paraxylene wherein the improvement comprises operating the process at a temperature in the range of from l60C to about 200C to thereby effect the desired separation of paraxylene under conditions resulting in low clution volume to feed ratios without a corresponding substantial decrease in the separation of ethylbenzene.

5. The process of claim 4 wherein said less preferentially adsorbed components are selected from the the separation of paraxylene contained in a C R aromatic isomer mixture containing ethylbenzene including the steps of:

group consisting of metaxylene, orthoxylene and ethylbenzene.

6'. The process of claim 4 wherein the eluent employed is metadiisopropylbenzene.

7. The process of claim 6 wherein the eluent strength is increased from a beginning 2571 concentration to a final concentration of up to about 8. The process of claim 4 wherein the carrier is selected from the group consisting of hexane. heptane.

isooctane cetane and eicosane. 

1. IN A LIQUID CHROMATOGRAPHIC SEPARATION PROCESS OF UP TO TWO COMPONENTS CONTAINED IN ADMIXTURE IN A FEEDSTREAM IN CLUDING THE STEPS OF: A. CONTACTING THE FEEDSTREAM MIXTURE, WHEREIN SAID FEEDSTREAM MIXTURE IS A C8 AROMATIC FEEDSTREAM CONTAINING ETHYLBENZENE WITH A BED OF CRYSTALLINE ALUMINOSILICATE ADSORBENT AT CONDITIONS TO EFFECT THE SELECTIVE RETENTION OF B. PASSING THROUGH SAID BED AN ELUENT SELECTED FROM THE GROUP CONSISTING OF AROMATICS AND SUBSTITUTED AROMATICS, C. INCREASING THE STRENGTH OF THE ELUENT DURING THE OPERATION OF STEPS (B) ABOVE TO THEREBY EFFECT DESIRED SEPARATION UNDER CONDITION THAT CAUSE LOW ELUTION VOLUME TO FEED RATIOS, D. RECOVERING FROM SAID BED STREAM OR STREAMS CONTAINING A PORTION OF THE LESS PERFERENTIALLY ADSORBED COMPONENTS AND E. RECOVERING A STREEAM OR STREAMS SUBSTANTIALLY ENHANCED IN UP TO TWO OF THE COMPONENTS BY ADSORBENT, CONCENTRATION OF PARAXYLENE AND ETHYLBENZENE RELATIVE TO OTHER FEEDSTREAM COMPONENTS WHEREIN THE IMPROVEMENT COMPRISES OPERATING THE PROCESS AT TEMPERATURE OF FROM ABOUT 160*C TO ABOUT 200*C TO THEREBY EFFECT THE DESIRED SEPERATION OF ONE SAID PERFERENTIALLY ADSORBED COMPONETS UNDER CONDITIONS RESULTING IN LOWE ELUTION VOLUME TO FEED RATIOS WITHOUT A CORRESPONDING SUBSTANTIAL DECREASE IN THE SEPERATION OF THE OTHER OF SAID PERFERENTIALLY ADSORBED COMPONENTS.
 2. The process of claim 1 wherein the initial concentration of said eluent is about 25% and is increased to a final concentration in the range of from about 50 to about 100% during the operation of steps (b) and (c).
 3. In a liquid chromatographic process for the separation of paraxylene from a feedstream containing a C8 aromatic isomer mixture and ethylbenzene including the steps of: a. contacting said feedstream with a bed of crystalline aluminosilicate adsorbent at conditions to effect the selective retention of paraxylene by said adsorbent; b. passing through said bed an eluent selected from the group consisting of aromatics and substituted aromatics; c. increasing the strength of the eluent in a liquid carrier containing said eluent during operation of step (b) above; d. recovering from said bed a stream or streams containing a portion of the less preferentially adsorbed feedstream components; and e. recovering a stream or streams substantially enhanced in concentration of paraxylene wherein the improvement comprises operating the process at temperatures of from 160*C to about 200*C to thereby effect the desired separation of paraxylene under conditions resulting in low elution volume to feed ratios without a corresponding substantial decrease in the separation of ethylbenzene.
 4. In a liquid chromatographic separation process for the separation of paraxylene contained in a C8 aromatic isomer mixture containing ethylbenzene including the steps of: a. contacting a C8 aromatic isomer mixture with a bed containing a type Y molecular sieve selected from the group consisting of potassium Y, ammonium/potassium Y and barium potassium Y at conditions to effect the selective retention of paraxylene by said sieve; b. passing through said sieve an eluent selected from the group consIsting of metadiisopropylbenzene, paradiisopropylbenzene and toluene; c. increasing the strength of the eluent in the liquid carrier containing said eluent during the operation of step (b) above; d. recovering from said bed a stream or streams containing a portion of the feedstream components less preferentially adsorbed by said sieve; e. recovering a stream or streams substantially enhanced in concentration of paraxylene wherein the improvement comprises operating the process at a temperature in the range of from 160*C to about 200*C to thereby effect the desired separation of paraxylene under conditions resulting in low elution volume to feed ratios without a corresponding substantial decrease in the separation of ethylbenzene.
 5. The process of claim 4 wherein said less preferentially adsorbed components are selected from the group consisting of metaxylene, orthoxylene and ethylbenzene.
 6. The process of claim 4 wherein the eluent employed is metadiisopropylbenzene.
 7. The process of claim 6 wherein the eluent strength is increased from a beginning 25% concentration to a final concentration of up to about 95%.
 8. The process of claim 4 wherein the carrier is selected from the group consisting of hexane, heptane, isooctane, cetane and eicosane. 